Since metal bellows tubes are metal tubes with a bellows-shaped tube wall and permit expansion/contraction, deflection and bending, they are used for the purpose of absorbing displacements such as of thermal expansion/contraction, vibration, quake resistance, quake absorption, land subsidence and the like in, for example, movable piping of industrial equipment and apparatuses; plant piping in steel, petroleum refining, petrochemistry, power generation and other key industries, and the like.
A metal bellows tube (hereinafter also simply referred to as “bellows tube”) has conventionally been produced by superposing a plurality of discs, pressed into a specified sectional shape, and welding the inner peripheries and outer peripheries of mutually adjacent discs. However, because this method involves a large number of manufacturing steps and is not suitable for mass production, the metal bellows tube obtained is expensive, and, when a stress is applied onto the weld, rupture and cracking are likely to occur at the portion and the durability is not satisfactory. Thus, methods of producing a bellows tube from a metal tube (fundamental tube) have been developed. As examples of such methods, the hydraulic forming method, the elastomer forming method, the continuous die forming method and the like can be mentioned.
The hydraulic forming method is a method wherein circular molds are arranged at a constant gap in the outer periphery of a metal tube, and a liquid is filled in the tube in this state and the tube wall of the metal tube is pressurized to form it into a bellows form. In addition, the elastomer forming method is a method comprising inserting an elastic member (elastomer) into a predetermined part in a metal tube with the metal tube set between a forming die and a core metal, pressurizing the elastic member from both ends thereof (both ends in the longitudinal direction of the tube) to swell the predetermined site of the metal tube by the pressurized deformation force of the elastic member, and then pressure-forming the swollen portion in the forming die. This method (i.e. work) is repeatedly conducted while moving the forming part in the longitudinal direction of the metal tube to form a bellows. The continuous die forming method is a method wherein a metal tube is passed through a corrugation forming die fit to a die holder via a bearing inserted therebetween, to support the center of the tube and the center of the die eccentrically. The projection of the die is intruded into the metal tube, while rotating the corrugation forming die around the center of the tube, to continuously form a circumferential groove, forming the tube wall in a bellows form. Details of these methods are described in “Sosei To Kakou” (Journal of the Japan Society for Technology of Plasticity), Vol. 32, No. 366 (1991-7), pp. 818-823.
The use of metal bellows tubes has been further expanding in recent years, and accordingly there is expectation of further improvement of the flexibility of the metal bellows tube, particularly of further improvement of the durability in repeated bending deformation (that is, bending fatigue resistance). However, a technology that enables sufficient improvement of the bending fatigue resistance of the metal bellows tube has not yet been fully established.
On the other hand, as a future important use of the metal bellows tubes, there is a use wherein a high-pressure fluid such as the hydrogen fuel cell must be transported.
The hydrogen fuel cell is an apparatus wherein hydrogen gas supplied as a fuel from outside and oxygen (normally that in the atmosphere) are electrochemically reacted in said cell to generate electricity. Because only heat and steam are produced as by-products from the electrochemical reaction in said cell, the hydrogen fuel cell is drawing attention as a clean energy source that does not contaminate the global environment.
In particular, means of transportation such as passenger cars and buses equipped with the hydrogen fuel cell as a power source for driving force (what is called fuel cell vehicles) are important as next-generation means of transportation, and various technologies for their practical application have been developed (see, for example, Publication of Unexamined Japanese Patent Application No. 2003-086213).
For bringing fuel cell vehicles into practical application in society, there is a need for, in addition to technology for the hydrogen fuel cell itself mounted on the vehicle, equipment for supplying hydrogen gas to fuel cell vehicles, like gasoline stations. Such hydrogen gas supply equipment, under the alias of “hydrogen gas supply station”, “hydrogen station” and the like, is under investigation for practical applications.
The present inventor has investigated the above-described hydrogen gas supply equipment and found that the flexible hose for supplying hydrogen gas from a storage tank fixed to the supply equipment to the fuel cell vehicle has not yet been investigated fully and involves a problem. The problem arises from the fact that the hydrogen gas to be supplied is high pressure gas not found conventionally.
Conventionally, there were some cases wherein compressed natural gas was used as a fuel gas for vehicles, in which cases the gas pressure was about 20 MPa (200 atm).
In contrast, the hydrogen gas used for the hydrogen fuel cell is of higher pressures such as 25 MPa (about 250 atm) and 35 MPa (about 350 atm), and supply thereof at a superhigh pressure of 70 MPa (about 700 atm) is being considered for the future.
To supply hydrogen at such high pressure from a fixed gas tank to vehicles of various sizes (in addition, there is variation in vehicle stop position), a flexible tube, as a mediator, is necessary.
Flexible tubes conventionally used for supplying a high-pressure gas of about 20 MPa or so, are resin tubes reinforced by containing a metal wire and a metal bellows tube wherein elongation is limited by covering the tube with a metal braid.
However, for supplying high-pressure hydrogen gas used in hydrogen fuel cells, the above-described resin tube is undesirable because the hydrogen gas passes through the tube wall. Also, the above-described metal bellows tube cannot supply a superhigh-pressure gas, as high as 70 MPa, due to repeated expansion/contraction from gas filling and release. Repeated action of stress in the circumferential direction of the tube body, bending fatigue from repeated bending deformation of the tube, and the like, metal fatigue can occur in the bellows portion and the tube may break. Also, no improvement has been made in the coating of the metal braid, thus the tube may break.
These problems are problems that arise likewise, not only in the case of high-pressure hydrogen gas supply in the hydrogen fuel cell, but also in the supply of other superhigh-pressure fluids.